Lower extremity amputation can cause impairments in gait and balance due to the loss of proprioception and motor control. The asymmetries present during amputee gait are thought to be one of the major contributors to secondary musculoskeletal problems such as osteoarthritis. After lower extremity amputation, physical rehabilitation is necessary to achieve functional ambulation; however, no an evidence-based standard of care exists for individuals who undergo transtibial (TT) amputations. The objective of this application is to determine the effectiveness o gait training in an immersive virtual environment with variable terrain in people with transtibial amputation. Our custom designed Virtual Gait Retraining System (VGRS) is a virtual reality (VR) system which provides visual and motor challenges while treadmill walking. The VGRS immerses the user in a virtual world, provides additional feedback with an avatar and presents the user with challenging tasks such as inclines and stair climbing which are complemented by the movement of the treadmill. Our central hypothesis is that people with amputations will achieve greater improvements in balance, mobility and gait function using our custom designed VGRS as compared to current conventional treadmill gait training. The proposed research is needed because the VGRS, a new and innovative rehabilitative system, has not been tested on TT amputees. Furthermore, the benefits of standardized treadmill training for TT amputees have not been fully examined. This objective will be accomplished by addressing the following specific aims: 1). Compare the efficacy of VGRS to conventional treadmill training. 2) Evaluate the rate of improvement over the course of each training program (VGRS and conventional treadmill). We will conduct a randomized clinical trial with twenty TT amputee subjects comparing 6 weeks of training with the VGRS to 6 weeks of training using a conventional treadmill protocol used in prior research (Darter et al. 2013). We will use gait analysis, postural sway and standard balance and mobility instruments including the 6-Minute Walk Test, Four Square Step Test, the Berg Balance Scale and the Dynamic Gait Index to assess improvements in gait and mobility. We will evaluate subject pre- and post-training and at an interim point 3 weeks into the training sessions. Interim assessments will include temporal-spatial gait parameters, the 6-Minute Walk Test and the Dynamic Gait Index. We anticipate that the VGRS will deliver more effective training by creating a realistic and challenging environment with accurate visual perception of a motor task which is being simultaneously performed by the user. We will also gain an understanding of the rate of improvement for TT amputees during treadmill training. The research design will demonstrate the need for immersive virtual reality that includes a rich visual display and affords the opportunity to train on a variety of terrain conditions and mobility tasks to capture the natural range of adaptive variation in the real world to improve gait in transtibial amputees.